Method for fabricating an integrated optical isolator and a novel wire grid structure

ABSTRACT

A method for fabricating an integrated optical isolator includes depositing a wire grid material on a magneto-optical substrate and depositing a resist film on the wire grid material. The method further includes bringing a mold with a wire grid pattern on contact with the resist film and compressing the mold and resist film together so as to emboss the wire grid pattern in the resist film. The method further includes transferring the wire grid pattern in the resist film to the wire grid material on the magneto-optical substrate by etching.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to methods for fabricating integratedoptical isolators. More specifically, the invention relates to a methodfor forming a wire grid polarizer on a Faraday rotator and a method forsuppressing reflection of rejected polarization.

[0003] 2. Background Art

[0004] Fiber-optic communications systems have three major components.These include (1) a transmitter that converts electronic data signals tolight signals, (2) an optical fiber that guides the light signals, and(3) a receiver that captures the light signals at the other end of thefiber and converts them to electrical signals. For high-speed datatransmission or long-distance applications, the light source in thetransmitter is usually a semiconductor laser diode. The transmitterpulses the output of the laser diode in accordance with the data signalto be transmitted and sends the pulsed light into the fiber. Infiber-optic communications systems, some light may be reflected backfrom the fiber network. This back reflection affects the operation ofthe laser diode by interfering with and altering the frequency of thelaser output oscillations. For this reason, an optical isolator istypically provided between the laser diode and the optical fiber tominimize the back reflection from the fiber network.

[0005]FIG. 1 shows a prior art optical isolator 2 which comprises amagneto-optical material 4, called a Faraday rotator, sandwiched betweenan entrance polarizer 6 and an exit analyzer polarizer 8. The polarizers6, 8 are typically polarizing glass chips. The exit analyzer polarizer 8is set at 45° relative to the entrance polarizer 6. The Faraday Rotator4 and the polarizers 6, 8 are surrounded by a permanent magnet 10, whichapplies a magnetic field to the Faraday rotator 4. The magnetic field inconcert with the Faraday rotator 4 causes the plane of polarization ofthe incident beam 12 to rotate 45° within the Faraday rotator 4, thusallowing the incident beam 12 to pass through the exit analyzerpolarizer 8. The transmitted beam is indicated at 14. Any reflectedlight that travels in the reverse direction is first polarized at 45° bythe exit analyzer polarizer 8. The Faraday effect is non-reciprocal.Thus the light that passes through the Faraday rotator 4 is rotated anadditional 45° and is then blocked by the polarizer 6.

[0006] To ensure desired characteristics of the optical isolator 2, thepolarizers 6, 8 must be precisely aligned with Faraday rotator 4 so thatthe appropriate angle is formed between the polarizers 6, 8. Because ofthe alignment requirements, the assembly process of the optical isolator2 is somewhat labor-intensive. Some manufacturers use manual methods forassembly followed by soldering, gluing, or welding techniques to fix theindividual components in place. The materials used to fix the componentsin place present reliability problems in terms of micro movement of thecomponents in hostile operating conditions. U.S. Pat. No. 5,757,538issued to Siroki proposes a solution which includes forming polarizingwire grids on both surfaces of a Faraday rotator. The wire grids on thesurfaces of the Faraday rotator are used in lieu of the polarizing glasschips 6, 8. The proposal of forming wire grid polarizers on the Faradayrotator suggests that a non-manual/automated process is envisioned.However, the Siroki patent does not indicate how this is done.

[0007] Several techniques are available for forming wire grid structureson substrates. One technique known as photolithography involvestransferring a wire grid pattern on a photomask to a surface of thesubstrate. A metal is first deposited on the substrate. Then aphotosensitive material, called a photoresist, is applied on the metallayer. The photomask with the wire grid pattern is aligned with thesubstrate so that the pattern can be transferred to the photoresist.Once the photomask is aligned with the substrate, the photoresist isexposed through the pattern on the photomask with a high intensityultraviolet light. Mask patterning in the photoresist could be made, forexample, by exposing the photoresist to an interference pattern inducedby a He-Cd laser. See M. Koeda et al., “Production of Metallic Grating,”PAJ-06174907, Jun. 24, 1994. The pattern formed in the photoresist istransferred to the metal layer by etching, e.g., reactive ion etching orion milling.

[0008] Photolithography is widely used in fabricating wire gridpolarizers that operate in the mid infrared region (approximately 3 μmto 25 μm). However, photolithography has limited application in the nearinfrared region (approximately 0.75 μm to 3 μm) because the gridstructure in this region requires submicron features. Currently, thesmallest line width that can be made with photolithography isapproximately 0.2 μm. The commercial technology for fabricatingsubmicron patterns is electron beam lithography (“EBL”). EBL involvesscanning a beam of electrons across a surface covered with a resist filmthat is sensitive to those electrons. Other methods for fabricating wiregrid polarizers are disclosed in S. Kawakami and H. Tsuchiya,“Polarizing Element,” PGJ-61-16991, May 2, 1986, T. Katsuragawa et al,“Polarizer, Its Production and Display or Display Device Provided withPolarizer,” PAJ-10213785, Aug. 11, 1998, and Y. Sato, “Production ofGrid Type Polarizer,” PAJ-09090122, Apr. 4, 1997.

[0009] EBL is capable of forming fine patterns but is much slower andgenerally costlier than photolithography. U.S. Pat. 5,772,905 issued toChou discloses a cost-effective process for forming submicron featureson a substrate. The process, called nano-imprint lithography, isessentially an embossing technology. In nano-imprint lithography, apattern is formed on a mold by such methods as EBL and subsequentetching processes. A mold made in such fashion is used in the embossingprocess. The mold is brought into contact with a thin film carried on asurface of a substrate, e.g., a silicon wafer, so that the pattern onthe mold can be embossed on the thin film. The thin film layer comprisesa thermoplastic polymer, e.g., polymethyl methacrylate (PMMA). Duringthe embossing step, the thin film, the substrate, and the mold areheated to allow sufficient softening of the polymer. At this time,pressure is applied to emboss the polymer. After a period of time, theentire assembly is cooled below the glass transition temperature of thepolymer and the mold.

[0010] Two options are available after embossing the polymer dependingon whether the polymer film is carried directly on the surface of thesubstrate or on a metal layer deposited on the surface of the substrate.If the polymer film is carried directly on the surface of the substrate,the thin sections of the embossed polymer are removed, e.g., by oxygenetching, to expose the underlying substrate. After removing the thinsections, a metallic material is deposited on the exposed substrate.Then, a lift-off process is used to remove the remaining polymer fromthe substrate. If the polymer film is carried on a metal layer depositedon the surface of the substrate, an etching process such as reactive ionetching or ion milling is used to etch into the metal layer. Whileetching into the metal layer, the polymer on the metal layer is alsoremoved. If the polymer is not completely removed from the metal layerby the time the pattern is etched into the metal layer, a solvent may beused to remove the remaining polymer.

[0011] Before fabricating the mold used in the embossing process, thewire grid pattern on the mold is modeled to ensure that the performanceof the resulting wire grid polarizer is acceptable. There are severalmathematical models and expressions that can be used to determine theperformance of the wire grid polarizer with respect to transmission ofparallel and perpendicular electric fields of light. These mathematicalmodels could be based, for example, on Maxwell's theory, transmissionline theory, rigorous coupled wave analysis (“RCWA”), etc. Conventionalmodeling of wire grid polarizers using these methods are generallymetallic lines on transparent substrates which would produce usefulpolarizer. FIG. 2 shows an example of such a wire grid polarizer withparallel metallic lines 16 on a transparent substrate 18. In operation,the polarized light that is not transmitted through the substrate 18 isreflected off the metallic surfaces 16.

SUMMARY OF INVENTION

[0012] In one aspect, the invention relates to a method for fabricatingan integrated optical isolator which comprises depositing a wire gridmaterial on a magneto-optical substrate and depositing a resist film onthe wire grid material. The method further includes bringing a mold witha wire grid pattern in contact with the resist film and compressing themold and resist film together so as to emboss the wire grid pattern inthe resist film. The method further includes transferring the wire gridpattern in the resist film to the wire grid material on themagneto-optical substrate by etching.

[0013] In some embodiments, the wire grid material comprises a metallicmaterial. In some embodiments, the wire grid material comprises adielectric material sandwiched between two metallic materials. In oneembodiment, the metallic materials are selected from the groupconsisting of Al, Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, and W. In oneembodiment, the dielectric material is selected from the groupconsisting of Si, SiO₂, and GaAs.

[0014] Other aspects and advantages of the invention will be apparentfrom the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a schematic of a prior art optical isolator.

[0016]FIG. 2 is a schematic of a wire grid polarizer.

[0017]FIG. 3A illustrates the heating step of a process for forming wiregrid polarizer on an optical substrate in accordance with one embodimentof the invention.

[0018]FIG. 3B illustrates the embossing step of the process for formingwire grid polarizer on an optical substrate.

[0019]FIG. 3C illustrates the de-embossing step of the process forforming wire grid polarizer on an optical substrate.

[0020]FIG. 3D shows a wire grid polarizer formed on an opticalsubstrate.

[0021]FIG. 4A shows a Faraday rotator with wire grid polarizers formedthereon using the process illustrated in FIGS. 3A-3D.

[0022]FIG. 4B shows a Faraday rotator with two wire grid polarizersformed on one surface.

[0023]FIG. 5 shows an anti-reflective wire grid structure according toone embodiment of the invention.

DETAILED DESCRIPTION

[0024] Embodiments of the invention provide a process for forming a wiregrid polarizer on a Faraday rotator and a novel wire grid structurewhich suppresses reflection of rejected polarization. The process forforming the wire grid polarizer on a Faraday rotator is based onnano-imprint lithography. In general, the process involves fabricating amold with a wire grid pattern using EBL. It should be noted thatalthough EBL is slow and relatively expensive, the mold made using thisprocess can be used to replicate many wire grid polarizers. The patternon the mold is embossed on a polymer resist film that is applied on awire grid material on a substrate. The pattern in the polymer resistfilm is transferred to the wire grid material by etching. The disclosedprocess enables direct fabrication of the wire grid polarizer on aFaraday rotator. In particular, integrated optical isolators wire gridpolarizers that are capable of being used at the near infrared region orinfrared region can be produced at relatively low cost withoutsacrificing performance.

[0025] Various embodiments of the invention will now be described withreference to the accompanying figures. For convenience, the followingdescription is outlined into three principal sections, including Methodfor Forming a Wire Grid Polarizer on a Substrate, Integrated OpticalIsolator, and Novel Wire Grid Structure.

Method for Forming a Wire Grid Polarizer on a Substrate

[0026] FIGS. 3A-3D illustrate a process for forming a wire gridpolarizer on a substrate. Referring to FIG. 3A, the process starts withfabrication of a mold 20 having a wire grid pattern. In oneimplementation, the wire grid pattern is an array of parallel gridelements 21 spaced apart a predetermined distance and having apredetermined width and depth. A wire grid material 22 is deposited on asubstrate 24. In one embodiment, the wire grid material 22 comprises ametal having a high index of reflection, e.g., aluminum or silver. Inthe novel wire grid structure of the present invention, which would befurther discussed below, the wire grid polarizer material 22 comprisestwo layers of metal separated by a dielectric material.

[0027] For an optical isolator application, the substrate 24 is aFaraday rotator material, e.g., bismuth-substituted rare-earth irongarnet, and is preferably coated with an anti-reflective material 25prior to depositing the wire grid material 22. A resist film 26 isapplied on the wire grid material 22. The resist film 26 comprises athermoplastic polymer, e.g., PMMA. To avoid sticking, the thermoplasticpolymer is preferably incompatible with the mold 20 and/or the mold 20is coated with a releasing agent that is incompatible with thethermoplastic polymer. The mold 20 and the resist-coated substrate 24are brought into contact and heated to a temperature above the glasstransition temperature of the thermoplastic polymer. The mold 20 andsubstrate 24 are compressed together, as shown in FIG. 3B, for a periodof time in order to allow the grid elements 21 on the mold 20 tosufficiently penetrate the resist film 26. This process is referred toas embossing.

[0028] Later, the mold 20 and the resist-coated substrate 24 are cooledunder constant pressure. After the mold 20 and resist-coated substrate24 are sufficiently cooled to harden the resist film 26, the mold 20 isseparated from the resist-coated substrate 24, as shown in FIG. 3C. Thisprocess is referred to as de-embossing. The result of the process isduplication of the wire grid pattern (21 in FIG. 3A) in the resist film26. The grid pattern in the resist film 26 is then transferred to thewire grid material 22 by anisotropic dry etching, e.g., reactive ionetching or ion milling. The anisotropic etching process etches throughthe thin sections 30 of the resist film 26 first and then through thewire grid material 22. While etching through the thin sections 30 of theresist film 26 and the wire grid material 22, the thick sections 29 ofthe resist film 26 are also being etched. The initial thickness of theresist film 26, i.e., the thickness of the resist film 26 before theembossing step, should be selected to allow complete etching through thewire grid material 22 before or by the time the resist film 26 iscompletely removed. A subsequent solvent rinse or oxygen plasma exposurecan be used to remove any resist film 26 that remains after the etchingprocess. FIG. 3D shows the wire grid pattern transferred to the wiregrid material 22. This wire grid pattern acts as a wire grid polarizer.

[0029] In the examples which follow, a 5 mm square pattern is embossedon PMMA and then transferred to aluminum layer on a glass substrateusing the process described above. It should be clear, however, that theexamples presented below are intended for illustration purposes only andare not intended to limit the scope of the invention as otherwisedescribed herein.

EXAMPLE 1

[0030] In this example, aluminum is evaporated on a glass substrate. Thethickness of the aluminum film on the glass substrate is approximately440 nm. Diluted PMMA is then spin coated on the aluminum layer on theglass substrate. The thickness of the PMMA is approximately 690 nm. Amold with a grid pattern having a grid period of 570 nm and a depth of690 nm and the PMMA-coated substrate are heated to 110° C. It takesapproximately 3 minutes to stabilize the temperature of the mold and thePMMA-coated substrate. The mold and PMMA-coated substrate are compressedtogether. The contacting pressure between the mold and PMMA-coatedsubstrate is increased up to 67 MPa. The mold and the PMMA-coatedsubstrate are maintained at this contacting pressure for 1 minute andthen cooled for 3 minutes. After cooling, the mold is removed from thePMMA. A change in the color of the embossed PMMA with changes in viewingangle indicates that the grid pattern has been made on the PMMA. Theembossed PMMA and the underlying aluminum layer are dry-etched byBCl₃/Cl₂N₂ gas flow for 90 seconds. The resulting patterned aluminumlayer acts as a linear polarizer at the near infrared region. Thecontrast ratio and insertion loss are determined to be 42 dB and 2.5dB,respectively, at 1540 nm wavelength. The embossing of the grid patternon PMMA using the process outlined above takes less than 8 minutes. Forcomparison purposes, the same grid pattern is made on PMMA using EBL.The patterning process using EBL takes approximately 25 minutes.

EXAMPLE 2

[0031] In this example, aluminum is sputtered on a glass substrate. Thethickness of the aluminum film is approximately 590 nm. Diluted PMMA isthen spin coated on the aluminum layer on the glass substrate. Thethickness of the PMMA is approximately 690 nm. A mold with a gridpattern having a grid period of 570 nm and a depth of 690 nm and thePMMA-coated substrate are heated to 110° C. It takes approximately 3minutes to stabilize the temperature of the mold and substrate. The moldand PMMA-coated substrate are then compressed together with a pressureof 67 MPa. The mold and the PMMA-coated substrate are maintained at thiscontacting pressure for 1 minute and then cooled for 4 minutes. Aftercooling, the mold is removed from the PMMA. A change in the color of theembossed PMMA with changes in viewing angle indicates that the gridpattern has been made on the PMMA. The embossed PMMA and the underlyingaluminum layer are dry-etched by BCl₃/Cl₂N₂ gas flow for 120 seconds.The resulting patterned aluminum layer acts as a linear polarizer at thevisible wavelength. The contrast ratio is determined to be in a rangefrom 58 dB to 61 dB and the insertion loss is determined to be in arange from 2.5 dB to 3.1 dB at 1540 nm wavelength.

Integrated Optical Isolator

[0032]FIG. 4A shows an integrated optical isolator 31 which includes aFaraday rotator 32 with wire grid polarizers 34, 36. The Faraday rotator32 is made of a magneto-optical material. In one embodiment, the Faradayrotator 32 is a bismuth-substituted rare-earth iron garnet substratesuch as available under the Latching™ by Lucent Technologies. TheLatching™ garnet substrate operates without a bias magnet. In general,any garnet material can be used. In other words, the invention is notlimited to latching garnets. The Faraday rotator 32 is coated with ananti-reflective material 38, e.g., silica. The wire grids 34, 36 areformed on the anti-reflection coated Faraday rotator 32 using theprocess described above. With the anti-reflective coating, the wiregrids 34, 36 act as if they were unsupported, causing contrast toincrease. Layers of anti-reflective coating and wire grids can beapplied to the Faraday rotator 32 to further increase contrast ratio.FIG. 4B shows an example where two layers of wire grids 34, 40 areformed on the incident surface of the Faraday rotator 32.

[0033] Returning to FIG. 4A, the wire grids 36 are set at an angle,usually 45°, with respect to the wire grids 34. In operation, the wiregrid polarizer 34 polarizes the incident light. The polarized lightenters the Faraday rotator 32 and is rotated 45° within the Faradayrotator 32. The rotated polarized light then passes through the wiregrid polarizer 36. Any polarization light that does not enter theFaraday rotator 32 gets reflected back to the source (not shown) unlessthere is a mechanism in place to suppress the reflection. Thisdisclosure presents a novel wire grid structure that suppressesreflection of rejected polarization, as will be subsequently describedbelow. Because the wire grids 34, 36 are directly applied on theanti-reflective coated Faraday rotator 32, a monolithic structure iscreated where the integrated optical isolator 31 would be more robustand stable in performance when subjected to environmental extremes. Byusing the process described above, large substrates of garnet can beprocessed and then diced into individual optical isolator units, thusproviding economy of scale.

Novel Wire Grid Structure

[0034] As previously mentioned, conventional modeling of wire gridpolarizers are generally metallic lines on transparent substrates whichwould produce useful polarizer, but reflection of rejected polarizationcould be excessive. FIG. 5 illustrates an anti-reflective wire gridstructure 42 that suppresses reflection of rejected polarization. Theanti-reflective wire grid structure 42 is made of three layers ofmaterial 44, 46, 48. The bottom layer 44 and the top layer 46 comprise ametallic material. The middle layer 48 comprises a dielectric material.The bottom layer 44 polarizes the light entering the substrate 50 whilelayers 44, 46, 48 in concert suppress the polarized light vector that isreflected from the bottom layer 44. In operation, rejected polarizationis reflected from the bottom layer 44 and then backwards through themiddle layer 48 and top layer 46. A small amount of incident light isalso reflected from the top layer 46. The reflected incident light fromlayer 46, being 180° out of phase with the rejected/reflected light fromlayer 44, destructively interfere with each other. As will be shownbelow, the wire grid structure 42 can be designed such that the incidentlight cancels the rejected polarization in the middle layer 48.

[0035] The type of materials used in the layers 44, 46, and 48, thedimensions of the grid structure 42, i.e., the width w of the gridelements 52 and the thickness t₄₄, t₄₆, and t₄₈ of the layers 44, 46,and 48, and the grid period P are selected based on the desiredwavelengths of operation and other performance criteria. Of interest inthis application is to obtain a polarizer with high transmission(ideally >95%), low insertion loss (<2dB), high contrast ratio (>40 dB),and low reflection of the rejected polarization (<2%) at a particularwavelength, e.g., 1550 nm. To obtain a high contrast ratio, the metallicmaterial used in the bottom and top layers 44, 46 should be reflectiveat the wavelengths of interest. Examples of metallic materials with highindex at 1550 nm wavelength, include, but are not limited to, Ag, Al,Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, and W. For the middle layer 48, thedielectric material should be transparent at the desired operatingwavelengths. Examples of dielectric materials that are transparent inthe near infrared and infrared region include, but are not limited to,SiO₂, Si, and GaAs.

[0036] The wire grid structure 42 was modeled using RCWA. The relevantparameters for the analysis are grid period, duty cycle, and aspectratio. Duty cycle is the width w of the grid elements 52 divided by thegrid period P. The aspect ratio is the sum of the thicknesses of thelayers 44, 46, 48 divided by the width w of the grid elements 52. In theanalysis, the index of the substrate 50 is set at 2.345 and theoperating wavelength is set at 1550 nm. The objective in this analysiswas to find wire grid structures which minimize reflection of rejectedpolarization and have a contrast ratio greater than 40 dB and a maximumaspect ratio less than or equal to 6.

[0037] It should be clear that these constraints are not intended tolimit the scope of the invention. Different constraints can be set basedon the intended application of the wire grid structure 42. In general,the smaller the aspect ratio, the easier it is to fabricate the wiregrid structure 42.

[0038] The following are examples of wire grid structures with zeropercent reflection of rejected polarization. In the examples, thematerials for the layers 44, 46, 48 are preselected. Therefore, theresulting wire grid structures are not necessarily the global optimumstructures. Furthermore, different structures can be obtained byrelaxing or changing the wire grid design constraints mentioned above.

EXAMPLE 3

[0039] This example assumes that the bottom layer 44 is made ofaluminum, the middle layer 48 is made of silica, and the top layer 46 ismade of gold. The grid dimensions are as follows: thickness of thebottom aluminum layer 44 is 466 mn, thickness of the top gold layer 46is 47 nm, thickness of the middle silica layer 48 is 527 nm, and widthof the each grid element 52 is 210 nm. The aspect ratio is 5, the gridperiod is 396 nm, and the duty cycle is 0.53. The transmission is 85.2%and the contrast ratio is 52.4 dB.

EXAMPLE 4

[0040] This example assumes that the bottom layer 44 is made of gold,the middle layer 48 is made of silicon, and the top layer 46 is made ofaluminum. The grid dimensions are as follows: thickness of the bottomgold layer 44 is 412 mn, thickness of the top aluminum layer 46 is 16mn, thickness of the middle silicon layer 48 is 211 nm, and width ofeach grid element 52 is 147 nm. The aspect ratio is 4.3, the grid periodis 334 nm, and the duty cycle is 0.44. The transmission is 90.9% and thecontrast ratio is 40.2 dB.

EXAMPLE 5

[0041] This example assumes that the bottom layer 44 is made ofaluminum, the middle layer 48 is made of silica, and the top layer 46 ismade of aluminum. The grid dimensions are as follows: thickness of thebottom aluminum layer 44 is 433 nm, thickness of the top aluminum layer46 is 12 nm, thickness of the middle silica layer 48 is 524 nm, andwidth of the each grid element 52 is 235 um. The aspect ratio is 4.1,the grid period is 398 nm, and the duty cycle is 0.59. The transmissionis 84.7% and the contrast ratio is 59.7 dB.

EXAMPLE 6

[0042] This example assumes that the bottom layer 44 is made of gold,the middle layer 48 is made of silica, and the top layer 46 is made ofgold. The grid dimensions are as follows: thickness of the bottom goldlayer 44 is 439 nm, thickness of the top gold layer 46 is 33 nm,thickness of the middle silica layer 48 is 512 nm, and width of the eachgrid element 52 is 191 nm. The aspect ratio is 5.1, the grid period is319 nm, and the duty cycle is 0.6. The transmission is 82.8% and thecontrast ratio is 59.8 dB.

[0043] The examples above show that the wire grid structure 42 iseffective in suppressing reflection of rejected polarization. The wiregrid structure 42 can be fabricated on a Faraday rotator material usingthe process described above. This would involve depositing a bottomlayer of metal on the Faraday rotator material, followed by a middlelayer of dielectric material, and a top layer of metal. Preferably, theFaraday rotator material is coated with an anti-reflective coating priorto depositing the bottom layer of metal. The dimensions of the wire gridstructure can be selected using appropriate modeling techniques such asRCWA. It should be noted that reactive ion etching should not be used totransfer the wire grid pattern into the wire grid material if any of themetal layers is made of gold or other similarly sensitive material.

[0044] The invention provides general advantages. Using the processdescribed above, wire grid polarizers with submicron features can beformed on a Faraday rotator to make an integrated optical isolator. Theanti-reflective wire grid structure suppresses reflection of rejectedpolarization, thus enhancing the performance of the optical isolator.

[0045] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for fabricating an integrated opticalisolator, comprising: depositing a wire grid material on amagneto-optical substrate; depositing a resist film on the wire gridmaterial; bringing a mold with a wire grid pattern in contact with theresist film and compressing the mold and resist film together so as toemboss the wire grid pattern in the resist film; and transferring thewire grid pattern in the resist film to the wire grid material on themagneto-optical substrate by etching.
 2. The method of claim 1, whereinthe wire grid material comprises a metallic material.
 3. The method ofclaim 1, wherein the wire grid material comprises a dielectric materialsandwiched between two metallic materials.
 4. The method of claim 3,wherein the metallic materials are selected from the group consisting ofAl, Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, and W.
 5. The method of claim 3,wherein the dielectric material is selected from the group consisting ofSi, SiO₂, and GaAs.
 6. The method of claim 1, wherein the resist filmcomprises a thermoplastic polymer.
 7. The method of claim 6, furthercomprising heating the mold and the resist film to a temperature abovethe glass transition temperature of the thermoplastic polymer prior tobringing the mold in contact with the resist film.
 8. The method ofclaim 1, further comprising coating the magneto-optical substrate withan anti-reflective material prior to depositing the wire grid materialon the substrate.
 9. A method for fabricating a wire grid polarizer,comprising: depositing a wire grid material on a substrate, wherein thewire grid material comprises a dielectric material sandwiched betweentwo metallic materials; depositing a resist film on the wire gridmaterial; bringing a mold with a wire grid pattern in contact with theresist film and compressing the mold and resist film together so as toemboss the wire grid pattern in the resist film; and transferring thewire grid pattern in the resist film to the wire grid material on thesubstrate by etching.
 10. The method of claim 9, wherein the metallicmaterials are selected from the group consisting of Al, Au, Cu, Ir, Mo,Ni, Os, Pt, Rh, and W.
 11. The method of claim 9, wherein the dielectricmaterial is selected from the group of Si, SiO₂, and GaAs.
 12. Themethod of claim 9, wherein the resist film comprises a thermoplasticpolymer.
 13. The method of claim 12, further comprising heating themold, the resist film and the substrate to a temperature above the glasstransition temperature of the thermoplastic polymer prior to contactingthe mold with the resist film.
 14. The method of claim 9, furthercomprising applying an anti-reflective coating on the substrate prior todepositing the wire grid material on the substrate.
 15. The method ofclaim 9, wherein the substrate is made of a magneto-optical material 16.An integrated optical isolator, comprising: a magneto-optical substratehaving a first surface and a second surface, the first and secondsurfaces being coated with an anti-reflection material; a first wiregrid structure formed on the first surface, the first wire gridstructure being adapted to suppress reflection of rejected polarization;and a second wire grid structure formed on the second surface androtated an angle with respect to the first wire grid structure.
 17. Theintegrated optical isolator of claim 16, wherein the first wire gridstructure comprises a plurality of substantially parallel grid elements,each grid element comprising a dielectric material sandwiched betweentwo metallic materials.
 18. The integrated optical isolator of claim 17,wherein the metallic materials are selected from the group consisting ofAl, Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, and W.
 19. The integrated opticalisolator of claim 17, wherein the dielectric material is selected fromthe group of Si, SiO₂, and GaAs.
 20. A wire grid polarizer, comprising:a substrate which is transparent at a selected wavelength; and ananti-reflective wire grid structure formed on a surface of thesubstrate.
 21. The wire grid polarizer of claim 20, wherein the surfaceof the substrate on which the anti-reflective wire grid structure isformed is coated with an anti-reflective material.
 22. The wire gridpolarizer of claim 20, wherein the anti-reflective wire grid structurecomprises a plurality of substantially parallel grid elements, each gridelement comprising a dielectric material sandwiched between two metallicmaterials.
 23. The wire grid polarizer of claim 22, wherein the metallicmaterials are selected from the group consisting of Al, Au, Cu, Ir, Mo,Ni, Os, Pt, Rh, and W.
 24. The wire grid polarizer of claim 22, whereinthe dielectric material is selected from the group of Si, SiO₂, andGaAs.